Secondary batteries are regarded with much interest as a power source of electric vehicles (EVs), hybrid electric vehicles (HEVs), and parallel hybrid electric vehicles (PHEVs), which are expected to reduce air pollution caused by typical gasoline and diesel vehicles using fossil fuels. Medium-to-large devices, such as vehicles, require high power and high capacity, and thus, employ a medium-to-large battery module that is formed by electrically connecting a large number of battery cells.
To optimally reduce the size and weight of medium-to-large battery modules, prismatic-type batteries and pouch-type batteries, which have high integration and a small weight-to-capacity ratio, are widely used as battery cells in medium-to-large battery modules.
An electrode assembly is accommodated in a case of a battery cell. Electrode assemblies may be classified according to types of structures including cathodes, separators, and anodes.
For example, electrode assemblies may be classified into jelly-roll (winding type) electrode assemblies having a structure formed by winding long sheet type cathodes and anodes with a separator therebetween, stacked type electrode assemblies formed by sequentially stacking a plurality of cathodes and anodes cut to a predetermined size with a separator therebetween, and stack and folding type electrode assemblies.
Stack and folding type electrode assemblies disclosed in Korean Patent Publication Nos. 2001-0082058, 2001-0082059, and 2001-0082060, applied by the applicant of the present invention, will now be described.
Referring to FIG. 1, an electrode assembly 1 having a stack and folding type structure includes: a plurality of full cells 1a, 1b, 2, 3, and 4, as unit cells, which are formed by sequentially stacking a cathode, a separator, and an anode and overlap one another; and a separator sheet 5 disposed between overlap parts of the full cells 1a, 1b, 2, 3, and 4. The separator sheet 5 has a unit length to surround a full cell and is bent inward by the unit length between the overlap parts to surround each full cell in a range from the full cell 1b disposed in the center of the electrode assembly 1 to the full cell 4 disposed on an outermost side of the electrode assembly 1. A distal end of the separator sheet 5 is finished by using heat welding or attaching an adhesive tape 6 thereto. Such stack and folding type electrode assemblies are manufactured, for example, by arraying the full cells 1a, 1b, 2, 3, and 4 on the separator sheet 5 having a long length and sequentially winding the full cells 1a, 1b, 2, 3, and 4 from an end of the separator sheet 5. However, under this structure, a temperature gradient is formed between the full cells 1a, 1b, and 2, disposed in the central region of the electrode assembly 1, and the full cells 3 and 4 disposed in outermost regions of the electrode assembly 1, and thus, heat dissipation efficiency varies therebetween, which decreases the service life of the electrode assembly 1 when being used for a long time.
A process of forming such electrode assemblies employs two lamination apparatuses for forming each of the electrode assemblies, and a separate folding apparatus. Thus, there is a limit in reducing a tact time of the process. Specifically, when a stacked structure is formed through folding, it is difficult to accurately align upper and lower electrode assemblies of the stacked structure, which makes it difficult to form an assembly having a reliable quality.
That is, a structure of electrode assemblies, to which such a folding process is applied, requires a separate folding apparatus. In addition, when a bi-cell structure is used, two types of bi-cells (that is, an A type bi-cell and C type bi-cell) are manufactured and stacked, and it is significantly difficult to accurately maintain a distance between bi-cells disposed on a long separator sheet before a folding process. That is, it is difficult to accurately align upper and lower unit cells (full cells or bi-cells) in a folding process. In addition, when a high capacity cell is manufactured, it takes a long time to change molds.
Next, stacked type electrode assemblies will now be described. Since stacked type structures are well known in the art, limitations of stacked type electrode assemblies will now be described briefly.
Horizontal and vertical widths of a separator of stacked type electrode assemblies may be greater than those of an electrode. Such a stacked type electrode assembly is manufactured by repeatedly performing a process of placing a separator on a magazine or jig having a width corresponding to the horizontal or vertical width of the separator, and placing an electrode on the separator.
However, in this case, electrodes and separators are stacked one by one, and thus, a working time is increased so as to significantly decrease productivity. While the separators can be horizontally and vertically aligned, there is no magazine or jig for accurately aligning the electrodes placed on the separators. Thus, electrodes of stacked type electrode assemblies may be misaligned.
Furthermore, since face-to-face surfaces of a cathode and an anode with a separator therebetween are misaligned from each other, an electrochemical reaction may not occur on one portion of active materials applied to the face-to-face surfaces, thus decreasing efficiency of a battery cell.